The present invention generally relates to timing circuits and more particularly to a proportional warmup timer capable of automatically adjusting the delay time to provide only the warmup time necessary should there be a short outage of line power causing only partial cooling of a piece of equipment.
There are many pieces of equipment which when power is initially turned on or restored after a power line outage must be prevented from operating until the equipment reaches proper operating temperature. Exemplary of such equipment is the power supply of a traveling wave tube amplifier used in the transmitter of microwave communication systems. Timers have been developed which are responsive to the turnon of power to delay the application of high voltage to the amplifier until a sufficient time has elapsed for the equipment to attain proper operating temperature.
One known warmup delay timer is an electromechanical unit. In this unit, an electric motor shaft speed is greatly reduced with gearing to provide a very slow motion to operate a switch. The shaft also winds a spring thereby storing energy. When power is removed, the spring starts to return the shaft to its original position. Should the power be restored before the shaft reaches its original position, the time of delay is reduced resulting in a delay proportional to the time that power was removed from the equipment. This is obviously very beneficial in equipment in which reliability is of prime importance. In the example of microwave communications systems, any equipment outage is at best a serious inconvenience. Therefore, it is advantageous to minimize the equipment outage which results from a power line outage.
While the electromechanical unit has been useful as a proportional warmup timer, it suffers several disadvantages. First of all, the unit is large and relatively unreliable especially when compared with modern integrated circuitry. Because it is an electromechanical unit, it lacks flexibility making it impossible to provide proportionality of restoration time for some periods of the delay and not for others. The spring used to provide a source of stored energy to run the unit after a loss of power is both large and unreliable.